U.S. patent application number 13/907262 was filed with the patent office on 2014-12-04 for corrosion resistant air preheater with lined tubes.
The applicant listed for this patent is Joe Ferguson, Brian Schifler, Steve Turner. Invention is credited to Joe Ferguson, Brian Schifler, Steve Turner.
Application Number | 20140352931 13/907262 |
Document ID | / |
Family ID | 51983806 |
Filed Date | 2014-12-04 |
United States Patent
Application |
20140352931 |
Kind Code |
A1 |
Turner; Steve ; et
al. |
December 4, 2014 |
Corrosion Resistant Air Preheater with Lined Tubes
Abstract
A dew point corrosion resistant heat exchanging system having a
plurality of hollow heat transferring tubes through which cooler
ambient air or hot combustion product gasses flow. The other of the
air or gas flows across the outer surfaces of the tubes, and heat
is transferred from the hot gasses to the ambient air, thus heating
the air. A portion of the tubes includes an inner liner forming an
air pocket chamber between the liner and the outer wall of the
tube. The air pocket chamber provides heat transfer advantages that
maintain the tubes at a temperature above the dew point of the
gasses in the system, thus inhibiting corrosion of the tubes.
Inventors: |
Turner; Steve; (Wheaton,
IL) ; Ferguson; Joe; (Robinson, IL) ;
Schifler; Brian; (Sugar Grove, IL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Turner; Steve
Ferguson; Joe
Schifler; Brian |
Wheaton
Robinson
Sugar Grove |
IL
IL
IL |
US
US
US |
|
|
Family ID: |
51983806 |
Appl. No.: |
13/907262 |
Filed: |
May 31, 2013 |
Current U.S.
Class: |
165/134.1 |
Current CPC
Class: |
F28F 21/082 20130101;
F28D 21/001 20130101; F23L 15/04 20130101; Y02E 20/348 20130101;
F28F 9/26 20130101; F28F 2270/00 20130101; F28D 7/1623 20130101;
Y02E 20/34 20130101; F28F 19/002 20130101; F28F 19/00 20130101 |
Class at
Publication: |
165/134.1 |
International
Class: |
F23L 15/04 20060101
F23L015/04 |
Claims
1-19. (canceled)
20. A durable dew point corrosion resistant heat exchanging system
for transfer of heat between a first gas and a second gas,
comprising: a. a plurality of hollow heat transferring tubes
aligned parallel to each other in the direction substantially
perpendicular to a first gas inlet, said heat transferring tubes
extending between said first gas inlet and a first gas outlet; b.
said plurality of hollow heat transferring tubes adapted to direct
said second gas through said hollow heat transferring tubes; c. a
first set of said plurality of heat transferring tubes having an
inner liner inside each said first set of said heat transferring
tubes, said inner liners each being sealed to a corresponding heat
transfer tube at both ends of each said inner liner, each said
inner liner spaced from an inner wall of each of said heat
transferring tubes, and forming a space between the inner liner and
the inner wall of the corresponding heat transferring tube, said
space defining a sealed fluid chamber; d. said sealed fluid chamber
extending through a first tube sheet; e. each said inner liner
having a first predetermined axial length; and f. said first set of
said plurality of heat transferring tubes including said first set
of lined hollow heat transferring tubes is arranged adjacent to the
first gas inlet, said first set of heat transferring tubes
extending for a second predetermined length into said heat
exchanging system in a direction along a path of said first gas,
said second predetermined length being determined by the
temperature of the inner liner remaining above a predetermined
temperature.
21. The heat exchanging system of claim 20, wherein said fluid
chamber extends through a second tube sheet.
22. The heat exchanging system of claim 21, wherein each of said
heat transferring tubes connects and extends through said first
tube sheet and said second tube sheet.
23. The heat exchanging system of claim 20, wherein said first gas
is ambient air and said second gas is hot exhaust gas from a
combustion heating system, said first and second gases having
different temperatures, heat being exchanged between said first gas
and said second gas.
24. The heat exchanging system of claim 23, wherein said heat
exchanging system comprises an air forcing apparatus to force said
first gas to said first gas inlet.
25. The heat exchanging system of claim 23, wherein said first path
of said first gas comprises a single air path.
26. The heat exchanging system of claim 25, wherein said second
predetermined length constitutes at least one third of the length
of the single air path in the direction of said single air
path.
27. The heat exchanging system of claim 23 wherein said first path
of said first gas comprises multiple interconnected air paths.
28. The heat exchanging system of claim 27 wherein said second
predetermined length constitutes at least one third of the length
of a first path of said multiple air paths in the direction of said
first gas path.
29. The heat exchanging system of claim 27 wherein said first
predetermined axial length of each said inner liner extends through
the entire width and perpendicular to a first path of said multiple
air paths, and extending a third predetermined length along the
axial direction of said hollow heat transferring tubes beyond said
first path and partially into a second path of said multiple air
paths.
30. The heat exchanging system of claim 29 wherein each said inner
liner extending along the third predetermined length prevents a
heat sink from forming at the second path of said multiple air
paths, said third predetermined length is in the range of twelve
inches to four feet.
31. A durable dew point corrosion resistant heat exchanging system
for transfer of heat between hot exhaust gas and a cool ambient
air, comprising: a. a plurality of hollow heat transferring tubes
aligned parallel to each other in the direction substantially
perpendicular to a hot exhaust gas inlet, said heat transferring
tubes extending between said hot exhaust gas inlet and a hot
exhaust gas outlet; b. said plurality of hollow heat transferring
tubes adapted to direct said cool ambient airthrough said hollow
heat transferring tubes; c. a first set of said plurality of heat
transferring tubes having an inner liner inside each tube of said
first set of said heat transferring tubes, said inner liners each
being sealed to a corresponding heat transferring tube at both ends
of each said inner liner, each said inner liner spaced from an
inner wall of each of said heat transferring tubes and forming a
space between the inner liner and the inner walls of the
corresponding heat transfer tube, said space defining a sealed
fluid chamber; d. said sealed fluid chamber extending through a
first tube sheet; e. said first set of said plurality of heat
transferring tubes including said first set of lined hollow heat
transferring tubes is arranged adjacent to the hot exhaust gas
outlet, said first set of lined tubes extending in a direction
towards said hot exhaust gas inlet; and f. said first set of said
plurality of heat transferring tubes extending for a first axial
predetermined length from said first tube sheet in a direction of
the cool ambient air flow, and extending a second predetermined
length from said first tube sheet in a direction opposite the
direction of the cool ambient air flow.
32. The heat exchanging system of claim 31, wherein said first
predetermined length is determined by a temperature of said hollow
heat transferring tubes remaining above a predetermined
temperature.
33. The heat exchanging system of claim 31, wherein said first
predetermined length along said hollow heat transferring tubes with
said inner liners is at least one third of a length of said heat
exchanging hollow tubes.
34. The heat exchanging system of claim 31 wherein said second
predetermined length of said inner liner is approximately four
inches.
35. The heat exchanging system of claim 31, wherein said system
further comprises an air forcing apparatus to force said cool
ambient air to said plurality of hollow heat transferring tubes
with inner liners.
36. The heat exchanging system of claim 31, wherein said first tube
sheet is protected by a heat insulation layer.
37. The heat exchanging system of claim 20, wherein said first gas
inlet comprises a bypass controlling the volume of said first gas
directed into said heat exchanging system.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to the structure and design of
an apparatus providing heat exchange between relatively hot gas and
relatively cold air in air preheaters for a fired burner. More
specifically, the invention is concerned with heat transfer from
the hot exhaust gasses that contain corrosive content, such as
sulfur and other chemicals, to the cool air while preventing
corrosion of the metal heat transferring tubes. The heated air is
utilized for combustion purposes.
BACKGROUND OF THE INVENTION
[0002] An air preheater is a device generally designed to heat air
prior to using the air for combustion in a combustion fired heating
system, such as a boiler. The primary objective of an air preheater
is to increase the thermal efficiency of the process. Air
preheaters are commonly used in large boilers found in thermal
power stations producing electric power from, e.g. fossil fuels,
biomasses or waste. Preheating the air can be achieved by utilizing
the heat from the exhaust gasses in the flue. Air preheaters
recover the heat from the boiler flue gas which increases the
thermal efficiency of the boiler by adding heat to the combustion
air.
[0003] A tubular type of air preheater for use in steam generators
in thermal power stations consists of straight tube bundles which
extend through the gas outlet or air inlet ducting of the boiler,
which tubes are open at each end outside of the ducting. The tubes
are located inside the ducting, and the hot furnace exhaust gasses
pass through or around the preheater tubes, transferring heat from
the exhaust gas to the air inside the preheater. Ambient air is
forced by a fan through a first end of the ducting located at one
end of the preheater tubes, and the heated air emerges into another
set of ducting, which carries the heated air to the boiler furnace
for combustion.
[0004] The most common flow arrangement for the tubular air
preheater is counterflow with gas passing vertically through the
tubes and air passing horizontally in one or more passes outside of
and in contact with the tubes.
[0005] Generally known preheater units comprise a plurality of heat
exchange tubes that are placed horizontally in the flue gas duct.
The heat exchange units on different height levels are connected to
each other by air ducts located outside the flue gas duct. In other
configurations the flue gas flows are inside the heat exchange
tubes, and the heat exchange tubes are vertical.
[0006] In either configuration, the temperature of the ambient air
at the inlet side of the preheater unit is significantly lower than
at the air outlet side. The cold ambient air at the inlet side can
cause a considerable cooling effect, due to the heat transfer
coefficient of the air flow at the point of inflow being
substantially higher compared to the developed flow deeper in the
air duct. Moreover, the ambient air forced across the heat exchange
tubes is not substantially warmed at the point of inflow into the
heat exchanger.
[0007] This strong cooling of the metal heat exchange tubes at the
air inlet can cause the surface of the heat exchange unit at the
inlet end to drop below the acid dew point of chemicals in the flue
gasses in contact with the tubes. One of the most serious problems
with tubular air preheaters is dew point corrosion. If the metal
temperature within the tubes drops below the acid saturation
temperature, usually between 190.degree. F. (88.degree. C.) and
230.degree. F. (110.degree. C.), but sometimes at temperatures as
high as 325.degree. F. (169.degree. C.), then the risk of dew point
corrosion damage to the tubes from the chemicals in the flue gas
becomes considerable. For example, the dew point of hydrochloric
acid (HCl) is around 175.degree. F., sulfuric acid
(H.sub.2SO.sub.4) is around 325.degree. F., and phosphoric acid
(H.sub.3PO.sub.4) is around 225.degree. F. The low temperatures
throughout the operating cycle create an extremely corrosive
environment for all the commonly used types of carbon steel tubes.
When the service life of the air preheater tubes falls to less than
five years, the operation and maintenance cost of the air preheater
dramatically diminishes the gross margin of the entire operating
facility.
[0008] Tube failures caused by high corrosion rates allow
combustion air to short-circuit the boiler and go directly up the
chimney. The induced draft and forced draft fan amperage is
increased to push/pull more air through the system until the boiler
capacity decreases due to the lack of combustion air reaching the
boiler. Also, all of the downstream pollution control systems are
negatively affected and the most, if not all, of the environmental
headroom is lost. Both of these effects can force the unit to be
taken off line to plug tube failures or clean the air heater. The
tube failures and fouling force the boiler to 1) burn more fuel; 2)
reduce the net electricity sold because the increased fan load
creates parasitic losses; 3) increase the amount of greenhouse
gasses entering the environment; and 4) decrease the gross margin
of the facility.
[0009] To overcome the described dew point corrosion problem, it is
common in the industry to either add more steam or gas air
preheaters upstream of the tubular air heater, or to substitute the
tube metallurgy to a more corrosion resistant material. If one
chooses to add preheaters upstream of the air heater, these
preheaters are typically used at startup and low load to increase
the air inlet temperature. In most air heaters the metal
temperature is above the acid dew point at full load. In the subset
of tubular air heaters, the acid dew point occurs downstream in the
pollution control systems. This method consumes significantly more
energy to preheat the incoming ambient air in the heat exchange
system.
[0010] The most common materials used to substitute for carbon
steel to make the heat exchange tubes are austenitic stainless
steel, and martensitic stainless steel. These stainless steels all
have low thermal conductivity. This causes the slow heating of the
metal tubes, causing the tubes to "sweat" and trap fly ash during
the operation of the system. The ashes can quickly fill the tubes,
eventually requiring removal of the tubes from service. The
austenitic stainless steels also have a high coefficient of thermal
expansion. This causes the heat exchange tubes to crack near the
tube sheets after repeated cycling. It is also known in the
industry to place insulation sleeves around the cold end of the
heat exchange tubes to prevent corrosion of the air pipe.
[0011] There lacks a durable apparatus and method for resolving the
dew point corrosion problem effectively and still maintain the
thermal efficiency of the heat exchanger in the system.
SUMMARY OF THE INVENTION
[0012] The present invention uses a low-cost, double-wall carbon
steel tubing structure to reduce corrosion rates and significantly
reduce fly ash fouling by raising the metal temperature of heat
exchanger tubes above the acid dew points of the corrosive species
present in the exhaust gasses. The system utilizes inner liners
together with the outer tube structure to change the thermodynamics
of the system and to maintain the temperature of the tubes above
the dew points of the corrosive acids. The thermal efficiency of
the system, however, is not significantly changed by using the
newly structured tubes. The present invention also saves energy and
improves the service life of the heat exchange tubes.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The features of the present invention which are believed to
be novel are set forth with particularity in the appended claims.
The invention may best be understood from the following detailed
description of currently illustrated embodiments thereof, taken in
conjunction with the accompanying drawings wherein like numerals
refer to like parts, and in which:
[0014] FIG. 1 is a vertical cross sectional view of one heat
transferring tube with inner liner.
[0015] FIG. 2 is a schematic view of an embodiment of an air
preheater where the air passes through multiple air paths.
[0016] FIG. 3 is a schematic view of another embodiment of the air
preheater where the air moves along a single air path.
[0017] FIG. 4 is a schematic view of another embodiment of the air
preheater where the hot exhaust gasses pass along the outside of
the metal tubes, and the air passes through the hollow tubes of the
preheater.
[0018] FIG. 5 is a detailed schematic illustration of the
connection between the tube sheet and several tubes in the heat
exchanger embodiment of FIG. 4.
[0019] FIG. 6 is a front cross-section view of the extension member
connecting two lined tubes.
DETAILED DESCRIPTION OF THE INVENTION
[0020] In a common counterflow heat exchange system, the hot gasses
and the ambient air to be heated are flowing in different
directions during the exchange heat process. The hot gasses come
into the system with a higher temperature and leave the system at a
lower temperature. The ambient air comes into the system with a
temperature lower than when the air leaves the system. A variety of
single and multiple gas and air path arrangements are employed to
accommodate plant layouts.
[0021] At the ambient air inlet end of the system in any of the
present designs for preheaters, both the ambient air forced into
the system and the hot exhaust gasses are at their lowest
temperature point in the entire process, which is normally below
the dew point for the corrosive chemicals in the exhaust gasses.
This is also where all the dew point corrosion is most likely to
take place. Also, in the scenario where the hot exhaust gas is
still above the dew point, the temperature of the metal tubes may
still be relatively cold. When the cold metal comes into contact
with the corrosive vapors in the exhaust gasses, vapors condense as
corrosive acid liquids to cause corrosion, and the liquid traps
ashes.
[0022] FIG. 1 shows a vertical cross sectional view of one heat
transferring tube 100 in accordance with the present invention with
inner liner 101. Having these liners 101 inside each of the heat
exchange tubes 100 maintains the temperature of the metal surface
of outer tube 103 above the dew point of the corrosive gasses,
thereby reducing corrosion of the tube 100. To reduce the
corrosion, the metal temperature of the heat transferring tubes
that directly contact the exhaust gas streams, for example the
inner liner 101, must remain above the dew point of the corrosive
gasses which flow through the tube 100, as shown by the arrow 104.
The inner liner 101 that is in direct contact with the exhaust gas
streams and the outer tube 103 are both made from metal materials
having high thermal conductivity, such as carbon steel. Air seals
105 at both ends of the inner liner 101, as explained with
reference to FIG. 6, seal a portion of air 107 between inner liner
101 and outer tube 103, forming an air pocket chamber or
intermediate layer in between the inner liner 101 and the outer
body 103 of the tube 100. The ambient air flows outside of the
outer body 103, as shown by arrows 106, and picks up heat by
contacting the outer metal surface of the outer tube 103. A
plurality of tubes 100 are held together in the heat exchanger by
means of one or more tube sheets 108.
[0023] The inner liner 101 passes heat from the hot gasses in path
104 through the sealed air portion 107 to the outer surface of tube
103 by radiation. The outer surface of tube 103 can be sufficiently
heated because of the high thermal conductivity of the materials
used for the liner 101 and outer tube 103. The ambient air outside
outer surface of the outer tube 103 picks up heat from contact with
the outer surface of outer tube 103, instead of directly drawing
heat from the inner liner 101. While the heat transfer from the
outer surface of the outer tube 103 to the ambient air would be
high, the presence of the intermediate layer of air 107 protects
the inner liner 101 from too high a rate of heat loss and super
cooling. This maintains the temperature of the metal surfaces of
liner 101 contacting the exhaust gas above the dew point
temperature without significantly dropping the thermal efficiency
of the heat exchange system.
[0024] FIG. 2 shows an embodiment of the air preheater system
utilizing the present invention. In this embodiment, the ambient
air to be heated is forced into the air preheater 200, usually by
an air fan. The ambient air is directed into the system through an
air inlet 202 and flows downward in accordance with the air flow
path defined by arrows 204. The ambient air path 204 in the present
embodiment has multiple interconnected paths. The cool ambient air
entering the multiple paths of this embodiment passes around and
over the tubes 212 several times through multiple paths downward.
The air is heated by the transfer of heat from the hot gasses in
tubes 212, and directed out of the system at air outlet 206,
compared with the single air path system shown in FIG. 3, where the
ambient air passes through a single path directly to the air outlet
306. In the embodiment of FIG. 2, when the cool ambient air flows
downward adjacent the tubes 212, the air in path 204 contacts the
outer surfaces of the heat transferring tubes 212 to pick up the
heat, such that the air is warmed when it arrives at the air outlet
206. While the ambient air in path 204 picks up heat from the outer
surfaces of tubes 212, the tubes are cooled by the flowing cool air
and loss of heat to the air as the air is warmed. The warmed air is
then supplied from outlet 206 as a source of oxygen for the
combustion chemical reactions in the boiler heating system, as is
known in the art.
[0025] The heat transferring tubes 212 are made from carbon steels
that have high thermal conductivity. The heat transferring tubes
212 are aligned parallel to each other in the direction
substantially perpendicular to the direction of air flow in air
inlet 202. A top tube sheet 214 and a bottom tube sheet 216 hold
the tubes 212 to maintain their positions. Each heat transferring
tube 212 has a very thin outer wall, and the thin walls form
conduits for the gasses to flow through the tubes 212. The hot
exhaust gasses 208 enter into the tubes 212 of the air preheater
200 from the top or bottom, and flow inside the tubes 212 to the
top of the preheater 200. The tubes 212 are heated as described in
conjunction with the tube 100 shown in FIG. 1 by contact with the
hot exhaust gasses 208.
[0026] There should be as rapid and as turbulent a flow of the
ambient air to pick up as much heat as possible from the metal
outer surfaces of the tubes 212. However, if the rapid heat
capturing causes the outer surface temperature of tubes 212 to drop
below the dew point, then, for example, the corrosive sulfur
trioxide which is present in the gas 208 will condense as sulfuric
acid and will cause corrosion. The outer surfaces of heat
transferring tubes 212 adjacent to the ambient air inlet 202 have
the highest rate of contact with the ambient cool air. Therefore,
the tubes 212 adjacent to the air inlet 212 are lined with liners
101 (FIG. 1) to protect the tube surfaces contacting the hot gasses
against corrosion.
[0027] Shaded area 210 in FIG. 2 illustrates where the inner liners
101 inside a plurality of heat transferring tubes 212 are located
in accordance with the present invention. In the illustrated
embodiment, all the cool air 204 entering into the system through
the air inlet 202 will contact the shaded portion 210 of the tubes
212. The cool air picks up heat from outer surface of the tubes 212
instead of directly from the inner liners 101 (FIG. 1), and the
inner liners 101 are protected from rapid heat loss and super
cooling due to the air pocket formed in sealed air portion 107. As
the cool air in path 204 picks up heat and the air flow is
eventually warmed up to a point such that the surface temperature
of the tubes 212 can be maintained above the dew point, the unlined
tubes or unlined portions of tubes 212, as shown in the brighter
areas in FIG. 2, are used to heat the air directly. To heat the
ambient air to the appropriate temperature above dew point, the
lined tubes 210, by calculation, should at least constitute one
third of the total numbers of the heat exchanging tubes 212 in the
first pass of air path 204, designated by shaded area 210.
[0028] The top portions of the liners 101 in the embodiment of FIG.
2 are also connected the top tube sheet 214. The vertical length of
the lined portion 210 of the tubes 212 extends at least no shorter
than the width of the first air path 204 to fully contact all the
cool air in path 204 entering the system through air inlet 202. The
liners 101 do not terminate just at the lowest point of the air
inlet 202, but extend further a short distance 220 toward the
bottom of the tubes 212. The temperature of the metal tubes 212 at
the bottom of distance 220 is above the dew point. The extended
portion 220 of the liners 101 is to prevent the joint of lined and
unlined portions of the tubes 212 adjacent to the lowest point of
the air inlet 202 from forming a heat-sink at the bottom of liner
101. This heat sink is to be avoided since the sink would take heat
from the metal surfaces of tubes 212, possibly causing the metal
tube temperature to drop below the dew point. By calculation and
experimentation, the liners 101 in the embodiment of FIG. 2 are
usually extended approximately 12 inches further down from the
lowest point of the first air path and into the second air path to
prevent forming of a heat-sink. In a heat exchanger segment length
of ten feet, the distance 220 could be four feet. An air bypass 218
may also be used to control the amount of cold air and further
prevent cold end corrosion.
[0029] FIG. 3 shows an embodiment of the present invention
installed in a single air path preheater system. In this
embodiment, the ambient air to be heated is forced into the air
preheater 300 through an air inlet 302 and advances through a
single path as illustrated by arrow 304 to the air outlet 306. When
the cool ambient air flows in single air path 304, the air contacts
outer surfaces of heat transferring tubes 312 to pick up the heat
from the flue gasses, such that the air is warmed above the dew
point when the air arrives at the air outlet 306. While the ambient
air picks up heat from the tubes 312, the tubes are cooled by the
flowing cool air. The warmed air is then supplied from outlet 306
as a source of oxygen for the combustion chemical reactions.
[0030] The hot exhaust gasses from boilers are directed into the
air preheater 300 through the top of a first set of the heat
exchanging tubes 312a as shown at the left side of FIG. 3. Arrow
308a in FIG. 3 shows the path for the hot gasses passing through
the system: from the top end of the first set of tubes 312a to the
bottom end of the first set of tubes, turning direction at the
corner chamber 310, then entering the bottom end of a second set of
tubes 312b, and finally exiting the system at 308b after passing
through the top end of second set of tubes 312b. The exhaust gasses
exiting the system 300 at 308b are cooled down as the heat from the
gasses passes through the walls of heat exchanging tubes 312 to
ambient air 304 flowing through the system.
[0031] Super cooling caused by rapid heat exchange can take place
at the air inlet end 302 of the heat exchange system of FIG. 3. To
prevent the temperature of the outer surfaces 103 (FIG. 1) of the
metal tubes 312 from dropping below the dew point, inner liners 101
(FIG. 1) are equipped in the heat exchanging tubes near the air
inlet 302 end of the preheater 300. Shaded area 314 in FIG. 3 shows
the placement of lined tubes.
[0032] As explained in the description of FIG. 2, all the cool air
coming into the system through the air inlet 302 in the embodiment
of FIG. 3 will contact the lined tubes 312 in area 314 first before
the air contacts unlined tubes 312 in the unshaded area. The cool
air in inlet path 302 picks up heat from the outer surfaces of the
lined tubes 312 in area 314, instead of directly from the inner
liners 101 (FIG. 1). Therefore, the inner liners in area 314 are
protected from rapid heat loss and super cooling. The inner
surfaces of inner liners 101, that contact the corrosive vapors
directly, are thus protected from corrosion, because the
temperature of the liners is prevented from suddenly dropping. As
the cool air advances through the system along path 304, the air
picks up heat and warms up to a point where the outer surface
temperature of the tubes 312a, b in the unshaded area can be
maintained above the dew point. In this manner, the unlined tubes,
as shown in the brighter areas in FIG. 3, are used to heat the air.
To heat up the ambient air to the appropriate temperature in the
embodiment of FIG. 3, the lined tubes in area 314, by calculation,
should at least constitute one third of the total number of heat
exchanging tubes 312a, b.
[0033] The vertical length of the lined tubes in area 314 are fully
extended no shorter than the width of air inlet 302 to fully
contact all the cool ambient air entering the system. The liners
101 (FIG. 1) used in the embodiment of FIG. 3 are held by the top
and bottom tube sheets 316 and 318. The inner liners 101 in the
system of FIG. 3 extend throughout the outer heat exchange tubes
312a, b as they are held by the top and bottom tube sheets 316 and
318.
[0034] FIG. 4 illustrates an embodiment of the heater 400 having
initial cool air, and ultimately warmer air passing through the
tube conduits 401, 403 and the hot exhaust gasses passing outside
the heat exchange tubes 412a, b located in the gas path 408. Heat
exchange tubes 412a, b in this embodiment form ambient air conduits
to initially direct cool air. Cool air is forced into an upper
portion 414 of lined tubes 412 (shaded) by an air forcing device,
such as an air fan. The air is heated after entering air inlet 402
due to contact with the upper portion 414 of the left end of the
heat exchange tubes 412a, b. The initially cool air travels through
the air preheating system and through tubes 412a, b along the path
depicted by arrows 404. After the cool air enters the system 400
through the air inlet 402, the air comes out from the first set of
heat transferring tubes 412a from the right end of the tubes, and a
cornered chamber 410 changes the air's direction of flow. The air
then enters the right end of the second set of tubes 412b, and
exits the system 400 from the left end of the second set of tubes
412b through the air outlet 406. The heat transferring tubes 412a,
b are held horizontally parallel to each other by a left tube sheet
416 and a right tube sheet 418.
[0035] Hot exhaust gasses 408 enter into the system in the
embodiment of FIG. 4 from the bottom and exit from the top. The
gasses can also enter from the top and exit from the bottom. The
hot exhaust gasses contact the outer surfaces of the tubes 412a, b
and transfer the heat to the tubes. The heat is then transferred to
the air inside the tubes and the air is heated. Dew point corrosion
can occur when the cooled gas at the top end of the heat exchanger
400 in FIG. 4 contacts the portion of tubes 412a adjacent the entry
of the initially cool air at air inlet 402.
[0036] All of the initially cool air forced into the heating system
400 by a fan passes through a portion of tubes 414 that are lined
(FIG. 1). Tubes 412a in portion 414 first become sufficiently
warmed by the hot gasses in path 404 to maintain the temperature of
the metal tubes 412a above the dew point of the exhaust gas
chemicals. The inner liners 101 (FIG. 1) are installed in the tubes
412a starting at the left end of the tubes, designated as portion
414, to receive the cold air. The inner liners 101 extend through a
predetermined length of the tubes in portion 414 in the direction
of the cool air's flow. A desired length of the liners in portion
414 of the embodiment in FIG. 4, by calculation, is approximately
one third of the total length of the heat exchanging tubes 412. At
this point, the temperature of the air in tubes 412a, b is
sufficiently higher than the dew point temperature. Each tube 412a
across the width of air inlet 402 is lined along the tube length
distance so described.
[0037] Additional protections are provided to prevent corrosion of
the cold end tube sheet 416 as well. Tube sheet 416 at the cold end
in the embodiment shown in FIG. 4 could create a heat sink, causing
corrosion at the joint of the tube sheet 416 and the tube 412a,
even if the tube sheet does not contribute to the heat exchange
between the cool air and hot gasses. When the cool air in path 404
contacts the tube sheet 416 adjacent air inlet 402, the tube sheet
can pull or drain the heat from the lined tubes in portion 414. The
intersection between the tubes portion 414 and the tube sheet 416
can be damaged because of the heat loss and contact with the hot
gasses in path 408. To prevent this damage, the lined tubes in
portion 414 extend outward from tube sheet 416 for a predetermined
length from the tube sheet 416, and the tube sheet 416 itself may
be covered by an insulation layer.
[0038] FIG. 5 illustrates the details of the tube sheet 416 in FIG.
4 showing the lined tubes 101 of portion 414 extending through the
tube sheet 416 for approximately four inches to receive the cold
air forced into the system from the air inlet 402 (FIG. 4). The
inner liners 101 also extend with the outer tube surface 103 (FIG.
1) a predetermine distance from the tube sheet 416. The space
between the inner liners 101 and the body of the outer surface 103
of tubes 100 are sealed at the left end 422 of the tubes 100 and
liners 101. Also, a layer of insulated refractory material 424 is
applied to the tube sheet 416, to further prevent heat loss from
and damage to the tubes 100 and tube sheet 416.
[0039] FIG. 6 shows schematically an extension member 601
frictionally engaging the abutted ends of two lined tubes 603 and
605 when it is necessary to combine two or more tubes to meet a
required length. Tubes 603 and 605 are the same as lined tube 101
of FIG. 1. The heat exchanging hollow tubes 603 and 605 are lined
with inner liners 607 and 609. Likewise, the air in chamber 604
between inner liner 609 and the hollow tube outer body 603 is
sealed at one end 611 adjacent the top tube sheet 613, and near the
opposite end by a seal 615. The air in chamber 606 between inner
liner 607 and hollow tube outer body 605 is sealed at one end by
seal 617 adjacent the other tube sheet 619, and near the opposite
end by seal 621.
[0040] To connect one end of tube 603 to an abutting end of tube
605, a hollow extension member 601 is partially and frictionally
inserted into one end of tube 603. The remaining part of the
extension member 601 is frictionally inserted into one end of tube
605. The connecting line 623 in FIG. 6 depicts where the two tubes
603 and 605 are in abutment and in contact with each other. The
hollow extension member 601 frictionally engages both the top tube
603 and the bottom tube 605 to maintain the tubes together to
achieve a desired length. The frictional engaging part of the
hollow connector 603 is optionally configured to allow the gas or
air to pass from hollow tube 603 to tube 605, while simultaneously
forming a seal between the outer surface of the connector 601 and
the inner walls of tubes 603 and 605.
[0041] While several particular embodiments of corrosive resistant
air preheaters of the present invention have been shown and
described, it will be apparent to those skilled in the art that
changes and modifications may be made without departing from the
true spirit and scope of the present invention. It is the intent of
the appended claims to cover all such changes and modifications as
fall within the true spirit and scope of the invention.
* * * * *